1. Technical Field
The present invention generally relates to methods and apparatus for processing received Orthogonal Frequency Division Multiplexing (OFDM) signals, and particularly relates to determining and updating decision boundary estimates used for generating soft bit values from QAM-modulated symbols.
2. Background
OFDM technology is a key component of the 3rd-Generation Partnership Project's (3GPP) “Long-Term Evolution” (LTE) initiative. Coupled with other evolving technologies, including Multiple-Input Multiple-Output (MIMO), an advanced antenna technology, the LTE initiative promises much higher data rates for mobile wireless users than are currently available, along with more efficient use of radio frequency spectrum.
As will be well understood by those skilled in the art, OFDM is a digital multi-carrier modulation scheme employing a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is separately modulated using conventional modulation techniques, including quadrature amplitude modulation (QAM). A QAM-modulated signal essentially comprises two carrier signals, in quadrature (i.e. ninety degrees out of phase with each other), which are separately amplitude modulated. A typical signal might use 16-QAM, which maps 4 bits to each symbol, or 64-QAM, which maps 6 bits to each symbol. Accordingly, an OFDM signal comprises a large number of sub-carriers, or “tones,” any or each of which may be separately modulated using QAM.
A transmitted OFDM signal undergoes various transformations and distortions as it travels through the propagation channel and is processed in a receiver. As a result, the absolute amplitude of any particular sub-carrier of the received OFDM signal at any given time is generally unpredictable. However, because QAM signals include an amplitude component, amplitude thresholds are needed to convert—“de-map”—the received QAM symbol into the original data bits. As a result, these amplitude thresholds, or “decision boundaries” must be derived from the received signal before the symbols can be de-mapped. Decision boundaries can be estimated from amplitude information derived from a number of received symbols.
Generally speaking, analyzing a larger number of received symbols can yield improved decision boundary estimates. However, analyzing a large number of received symbols requires that those symbols be stored until a decision boundary estimate is generated and the symbols are de-mapped. The system memory required for this storage is costly, in terms of increased integrated circuit size as well as increased power consumption.
In addition, because of frequency-selective effects of the propagation channel, symbols from a sub-carrier at one end of the OFDM signal may be of little use in estimating a decision boundary for symbols derived from a sub-carrier at the opposite end. Likewise, time-varying effects of the propagation channel will cause the amplitudes of the received signals to vary over time. These challenges complicate the design and implementation of efficient and robust decision boundary estimation algorithms.